Empty Queue should reset the InFlight count of consumers

This will keep them insync after they try and fail to FIN messages
that were InFlight when the Queue was Emptied.
Merge pull request #160 from dmarkham/empty_channel_testing

nsqd: Empty Queue should reset the InFlight count of consumers
flesh out production install details
Merge pull request #163 from mreiferson/install_doc

improved install docs